Rapid heating thermal shock study of ultra high temperature ceramics using an in situ testing method

Abstract In this paper, the rapid cooling thermal shock behaviors of ZrB2–SiC ceramics were measured using traditional water quenching method, and the rapid heating thermal shock behaviors of ZrB2–SiC ceramics were investigated using a novel in situ testing method. The measured critical thermal shock temperature difference for rapid cooling thermal shock was 373.6 °C; however, the critical thermal shock temperature difference for rapid heating thermal shock of ZrB2–SiC ceramics was measured to be as high as 1497.2 °C. The thermal stress distribution states after rapid cooling thermal shock and rapid heating thermal shock testing were analyzed using finite element analysis (FEA) method. The FEA results showed that there is a tensile stress existed on the surface for rapid cooling thermal shock, whereas there is a compressive stress existed on the surface for rapid heating thermal shock. The difference of thermal stress distribution resulted in the difference of the critical temperature difference for rapid cooling thermal shock and rapid heating thermal shock

Controlling fuel crossover and hydration in ultra-thin proton exchange membrane-based fuel cells using Pt-nanosheet catalysts

An ultra-thin proton exchange membrane with Pt-nanosheet catalysts was designed for a self-humidifying fuel cell running on H-2 and O-2. In this design, an ultra-thin Nation membrane was used to reduce ohmic resistance. Pt nanocatalysts were uniformly anchored on exfoliated, layered double hydroxide (LDH) nanosheets by chemical vapor deposition. After embedding Pt-LDH nanocatalysts in 9 mu m-thick Nation membranes, exfoliated LDH nanosheets effectively captured crossovered H-2 and O-2 through the membranes. Meanwhile, Pt nanocatalysts on LDH nanosheets catalyzed reactions between captured H-2 and O-2 and provided in situ hydration inside Nation membranes to maintain their proton conductivity level. Furthermore, LDH nanosheets reinforced the Nation membranes, with 181% improvement in tensile modulus and 166% improvement in yield strength. In a hydrogen fuel cell running with dry fuel, the membrane-electrode assembly employing the Pt-LDH/Nafion membrane showed an improvement of 200% in maximum power density, an increase of 197% in current density at 0.3 V and an improvement of 497% in current density at 0.5 V as compared to those with Nation 211. The Pt-LDH/Nafion membrane with a thickness of 9 mu m exhibited a combination of desirable properties for the development of affordable and durable hydrogen fuel cell technology, including better mechanical properties, higher open-circuit voltage, lower ohmic resistance and enhanced water management in a hydrogen fuel cell without external humidification

Quasi-static and dynamic mechanical properties of additively manufactured Al2O3 ceramic lattice structures: effects of structural configuration

Ceramic lattice structures (CLSs) are promising candidates for structural applications used in conventional and extreme environments because of their extraordinary properties. Herein, CLSs with different structural configurations, including body-centred cubic (BCC), Octet, and modified body-centred cubic (MBCC), were designed and fabricated by digital light processing (DLP)-based additive manufacturing technology to explore their quasi-static and dynamic compressive behaviours. It was demonstrated that when relative density was a constant, quasi-static compressive strength (QS), quasi-static Young’s modulus (QY), and quasi-static energy absorption (QE) of CLSs with an MBCC structural configuration were the best, Octet ranked secondly, BCC was the poorest. The same thing happened on dynamic mechanical properties of CLSs. Increasing the relative density from 20% to 40% dramatically improved the QS, QY, and QE of CLSs. Furthermore, it was revealed by experiment and simulation that the quasi-static failure mode of CLSs changed from partially fracture along a specific plane to integrally fracture at most nodes as relative density increased. Furthermore, the dynamical mechanical properties of CLSs were significantly outstanding than quasi-static mechanical properties due to the strain-rate effect. This study provides a new basis for further study on tailoring the mechanical properties of CLSs

Design and fabrication of porous ZrO₂</(ZrO₂+Ni) sandwich ceramics with low thermal conductivity and high strength

3Y-ZrO2/(3Y-ZrO2 + Ni) sandwich ceramics were fabricated through cold isostatic pressing and pressureless sintering. Porous 3Y-ZrO2 ceramics with large connecting open pores and permeability were used as interlayers for insulation, whereas outer metal-ceramic layers were used as bearing loads. Microstructures and properties of the porous ZrO2 and ZrO2/(ZrO2 + Ni) sandwich ceramics were investigated in detail. The ZrO2/(ZrO2 + Ni) sandwich ceramics exhibited better mechanical properties than the monolithic porous ZrO2 ceramics at the same low thermal conductivity (approximately 0.85 W/m K). The mechanical properties of the sandwich ceramics were influenced by metal toughening and sintering-induced residual thermal stress. (C) 2014 Elsevier Ltd. All rights reserved.Materials Science, MultidisciplinarySCI(E)[email protected]

The impact force identification of composite stiffened panels under material uncertainty

Aeronautical Science Foundation of China [2012ZA52001]; Specialized Research Fund for the Doctoral Program of Higher Education [20123218120005]This paper presents a synthesis approach to address the problem of uncertainty in the impact force identification. The effects of material uncertainty on dynamic responses of the structure are studied by using Monte Carlo simulation. Six parameters, including mechanical properties and thermal coefficients, are considered as independent random variables. A parametric study is conducted to select four parameters as the optimization variables in the following step of model updating. The technique of model updating is used to correct the modeling errors caused by material uncertainty. Then, an improved inverse analysis technique based on the finite element method and mode superposition method is taken for impact force identification. In this study, the present method is performed on a composite stiffened panel, and the effect of noise on the performance of identification is also discussed. The results of the study show that the developed approach is capable of identifying the impact location and reconstructing the force history accurately by reducing material uncertainty through the modal updating procedure. (C) 2013 Elsevier B.V. All rights reserve

Load distribution in threads of porous metal-ceramic functionally graded composite joints subjected to thermomechanical loading

Metal-ceramic functionally graded materials (FGMs) have been extensively used in aerospace engineering where high strength and excellent heat insulation materials are desired. In this paper, load distribution in threads of the Thermal Protection System used bolted joint made up of porous ZrO2/(ZrO2 + Ni) FGMs is investigated by ABAQUS codes. The bolted joint is subjected to reentry heating corresponding to the Access to Space Vehicle. Effects of bolt-nut parameters including thread tooth profile, thread pitch, and modulus ratio of bolt to nut on load distribution in threads are analyzed in detail. It is found that uneven load distribution in threads occurs at elevated temperature, which mainly focuses on the first two threads closest to the nut bearing surface, with the first thread carrying 74% of the total load. Bolt-nut parameters have great effects on load distribution in threads, with trapezoidal thread, extra fine thread and greater modulus ratio of bolt to nut leading to more evenly distributed load. Further studies show that nut shape has significant effects on load distribution in threads, the optimized nut is designed to make the maximum load bearing ratio of the thread decrease to 30.21%, and thus the service reliability of the bolted joint is greatly improved. (C) 2015 Elsevier Ltd. All rights reserved.National Natural Science Foundation of China [11090330, 11090331, 11102003, 11472038]; National Basic Research Program of China (973 Program) [2010CB832701, 2011CB610303]; Foundation of the Author of National Excellent Doctoral Dissertation of China [201029]SCI(E)[email protected]

Joining of Cf/SiC Ceramic Matrix Composites: A Review

Carbon fiber-reinforced silicon carbide (Cf/SiC) ceramic matrix composites have promising engineering applications in many fields, and they are usually geometrically complex in shape and always need to join with other materials to form a certain engineering part. Up to date, various joining technologies of Cf/SiC composites are reported, including the joining of Cf/SiC-Cf/SiC and Cf/SiC-metal. In this paper, a systematic review of the joining of Cf/SiC composites is conducted, and the aim of this paper is to provide some reference for researchers working on this field